1. Field of the Invention
This invention relates in general to disk storage systems, and more particularly to a method and apparatus for monitoring track misregistration.
2. Description of Related Art
Data storage systems typically include one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator and passed over the surface of the rapidly rotating disks.
The actuator typically includes a plurality of outwardly extending arms with one or more transducers being mounted resiliently or rigidly on the extreme end of the arms. The actuator arms are interleaved into and out of the stack of rotating disks, typically by means of a coil assembly mounted to the actuator. The coil assembly generally interacts with a permanent magnet structure, and the application of current to the coil in one polarity causes the actuator arms and transducers to shift in one direction, while current of the opposite polarity shifts the actuator arms and transducers in an opposite direction.
In a typical digital data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, closely spaced tracks comprising the surface of the magnetizable rigid data storage disks. Data is transferred to, and retrieved from, specified track locations by the transducers being shifted from track to track, typically under the control of a controller. The transducer assembly typically includes a read element and a write element. Other transducer assembly configurations incorporate a single transducer element used to write data to the disks and read data from the disks.
Writing data to a data storage disk generally involves passing a current though the write element of the transducer assembly to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer assembly sensing the magnetic field or flux lines emanating form the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals in the read element. The electrical signals correspond to transitions in the magnetic field.
Conventional data storage systems generally employ a closed-loop servo control system for accurately and rapidly positioning the actuator and read/write transducers to specified storage locations on the data storage disk. A servo writing procedure is typically implemented to initially record servo information on the surface of one or more of the data storage disks. A servo writer assembly is typically used by manufacturers of data storage systems to facilitate the transfer of servo data to one or more data storage disks during the manufacturing process. In accordance with one known servo information format, termed an embedded servo, servo information is written between the data storing sectors of each track. The servo data is thus embedded in the data storing tracks of the data storage disks, typically resulting in an alternating sequence of data and servo sectors comprising each track.
In accordance with another known servo information format employed in data storage systems, termed a dedicated servo, the servo writer records servo information typically on only one of the data storage disks comprising the disk stack, and often on only one of the surfaces of the dedicated servo disk. The servo information stored on the dedicated servo disk is used to maintain accurate positioning and alignment of the read/write transducers associated with each of the data storage disks. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as part of the read element of the transducer, is typically employed to read the servo sector data for the purpose of locating specified track and data sector locations on the disk. It is noted that a servo sector typically contains a pattern of data, often termed a servo burst pattern, used to maintain optimum alignment of the read/write transducers over the centerline of a track when reading and writing data to specified data sectors on the track.
Many factors affect the tracking position of a read-write head during both reading and writing on magnetic disk mediums. It is ideally desired that the read-write head be positioned exactly on the track position for each revolution of the disk during both the read and the write modes of operation. When such perfect track registration is attained, the signal amplitude of the desired data read-back is at a maximum. In such conditions, any newly written data will also occupy the exact same track region as did the old data which it replaces. However, in practical operating systems, the read-write head is very rarely aligned perfectly with the data track that exists. This gives rise to the phenomenon of track misregistration (TMR). Track misregistration is the result of many factors such as mechanical vibration, disk eccentricity or runout, bearing eccentricity and runout, servo system tracking errors and detection errors, etc. Erroneous data can be detected by the read head when significant track misregistration occurs.
It is a routine measurement to estimate the TMR for each head during manufacturing test. This measurement is accomplished by means of: 1) acquiring the PES signal form the drive; and 2) analysis of the PES data to estimate non-repeatable runout (NRRO), repeatable runout (RRO) and random transient vibration (RTV).
The TMR estimate as described has disadvantages. First, the test is costly in terms of test time. Secondly, because of the test time limitation, TMR measurements are restricted to the inner and outer most data cylinders. TMR estimation for the entire surface is interpolated form the ID/OD track measurements.
Pass/fail TMR specification based on this interpolation must assume worst case TMR to avoid potential shipment of a poor performing drive. If more track samples were practical, then a more optimized pass/fail criteria could be realized. This would eliminate the need to base the reject criteria on worst case assumptions and provide a more reliable indication of TMR performance.
Thirdly, the correlation with real customer environments is unclear because the TMR pretest is constrained to a particular test time. For example, thermal track shift in a particular customer environment would not be accounted for due to a difference in ambient temperature outside the file. This temperature difference results in track misregistration because the arms and the disk comprise different materials that have different thermal expansion coefficients.
Finally, the TMR pretest is useful for maintaining quality control during manufacturing. However, the TMR pretest obviously is not useful for determining in the field when a drive may need to be recalibrated to solve problems encountered in the field. For this scenario, an actual field TMR test is needed. However, files do not include means for performing a self-test of the file in the field.
It can be seen that there is a need for a method and apparatus for monitoring track misregistration that is quicker and that is not limited to worst case assumptions.
It can also be seen that there is a need for a method and apparatus for performing TMR self-test in the field.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for monitoring track misregistration.
The present invention solves the above-described problems by providing a method and apparatus for monitoring track misregistration that is quicker and that is not limited to worst case assumptions.
A system in accordance with the principles of the present invention includes a first memory for accumulating position error signals for a head to produce an accumulated value, a processor for normalizing the accumulated value to produce a normalized result at a predetermined trigger event and a second memory for adding the normalized result therein to produce a running sum; wherein the head is positioned using the running sum, the processor resetting the first memory when the normalized result is produced.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that a counter is provided for incrementing a count value after each position error signal is added to the first memory.
Another aspect of the present invention is that the processor normalizes the accumulated value according to the count value of the counter, and wherein the processor resets the counter as a result.
Another aspect of the present invention is that the processor normalizes the accumulated value according to a population mean and a variance associated with a profile of the system.
Another aspect of the present invention is that the trigger event includes the accumulated error value reaching a predetermined threshold.
Another aspect of the present invention is that the trigger event includes a servo seek to the next track.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.